In his excellent article "Hubble Trouble" {July 15} Stanley Rosen is correct in his argument that we can learn from the mistakes of the Hubble Space Telescope. But he misses perhaps its most significant lesson.

It appears to be generally recognized that the reason the spherical aberration that afflicts the Hubble telescope was not detected prior to launch was because the two mirrors were not tested together as a system. High cost is given as the justification for not doing so, and we are now paying the price for that in dollars and time. But the nation will survive, and the Hubble will in a few years be fitted with correcting lenses to sharpen its image of the universe. Some heads may roll, but no lives will be lost.

For the past several years many highly respected scientists and engineers have repeatedly pointed out that it would not be fundamentally possible to test a complete Strategic Defense Initiative system, especially in face of overwhelming countermeasures, short of an actual nuclear attack. The consequence of an error in the extraordinarily complex computer software or the exquisitely precise hardware required would not be paid for with dollars or time but with the lives of tens of millions of Americans.

The Hubble problems teach us to stop the delusion generated by the SDI zealots and to accept reality. Some $20 billion have already gone down the SDI drain. The $3 billion to $5 billion dollars budgeted for its next fiscal year can better be spent on more practical and essential military or domestic programs. ALBERT WEINSTEIN Alexandria